Base stations are conventionally used in contactless interrogation systems such as entrance doors which can be opened by bringing near them transponders such as entry cards. A base station essentially includes an oscillating control device, an amplitude modulating circuit and control means acting on an antenna connected to the base station. The oscillating control device is conventionally a peripheral device. The oscillating control device is generally a quartz crystal which supplies a control signal having a determined carrier frequency, when the quartz crystal vibrates at a frequency generally higher than the resonant frequency of the antenna (typically 1 MHz). The control signal is then modulated by the modulating circuit which also currently comprises a division chain for bringing the control signal frequency to the antenna resonant frequency (typically 125 MHz). Then this signal is transmitted by the antenna which is essentially formed of a coil. The transmitted modulated signal is capable of being received by a transponder if the latter is brought near the base station.
Transponders essentially do not have batteries and are intended to exchange information on data with a contactless interrogation device such as a base station of the aforementioned type. The supply voltage is supplied to the transponders from the control signal transmitted by the base station. This signal is modulated by the transponder, and the signal thus modified is intended to be read by the base station.
British Patent Application NO 95175626.1 discloses such a base station used in a vehicle anti-theft system.
A problem encountered with these base stations lies in the fact that the carrier frequency Fo of the modulated control signal has to be as close as possible to the antenna resonant frequency Fr1, and the transponder resonant frequency, Fr2. The effect of this is to increase the reading sensitivity of the base station.
Base stations according to the prior art do not provide a satisfactory solution to this problem. The dispersion of the values of all system components induces a notable difference between the antenna resonant frequency and the frequency supplied to the antenna. The base station reading sensitivity is linked to fluctuations in the voltage across the terminals of the antenna, which are themselves linked to variations in the signal received by the antenna. By way of illustration, FIG. 1a shows the voltage fluctuations .DELTA.V across the antenna terminals as a function of the frequency Fr1 and the frequency Fr2, frequency Fo being equal to 125 kHz, and FIG. 1b shows some isovalue curves (i.e. at constant .DELTA.V) associated with FIG. 1a. The frequency range within which the base station operates properly is limited to a few percent around the frequency Fo, as is better shown in FIG. 1b. Thus a difference between the transmitted signal frequency and the antenna resonant frequency leads to a loss of power during exchange with the transponder.
The presence of regions corresponding to negative .DELTA.V fluctuations, i.e. regions in which the data originating from the transponder is inverted, will also be noted in FIG. 1b. The base stations of this type of the prior art thus have an additional drawback. It is understood that the presence of the inversion regions may be particularly inconvenient when these base stations are used, for example, in an entrance door opening system.
Other solutions exist in the prior art, in particular base stations in which the components are discreet, complex and thus expensive. For example, the aforecited Patent Application discloses a contactless interrogation base station using a microprocessor as oscillating control device.